1. Field of the Invention
The present invention relates to a coil arrangement for measurements using magnetic resonance, especially nuclear magnetic resonance, having at least one coil configured as an axially slotted ring, for generating and/or receiving a HF measuring field extending substantially in parallel to the coil axis, the measuring object being located in a measuring space adjoining a first radial plane of the coil.
2. Description of the Prior Art
A coil arrangement of the before-described type has been known from U.S. Pat. No. 4,866,387.
The known coil arrangement, usually described as "CRC coil" (CRC=Counter Rotating Currents), consists essentially of two rings, which are arranged coaxially and at an axial distance relative one to the other and which are provided with one axial slot each at aligned circumferential positions. In the area of their axial slots, the two rings are designed as capacitors, for example in such a way that a copper strip constituting the ring is bent over in this area in radial direction in the form of two parallel legs.
The known coil arrangement may be operated in a basic oscillating mode in which the field lines of a magnetic alternating field pass through both coils. However, the coil arrangement may also be operated in a second, higher-frequency oscillating mode in which the HF currents flow in opposite senses in the circumferential direction of the two rings. The before-mentioned field lines will then pass through one of the two rings only and will leave the rings in opposite axial directions.
This measure is based on the following considerations:
It has been known that coils of the type used for example for nuclear resonance measurements, whose size is selected in such a way that the wavelength of the natural resonance is in the range of the measuring frequency, have an unfavorable signal-to-noise ratio. Now, it is of course imaginable to make the coils smaller, geometrically, which would automatically reduce their inductance, and to tune the coil so received thereafter to the desired operating frequency by applying suitable capacitors. This would indeed lead to an improvement of the signal-to-noise ratio, but would have to be paid for by a simultaneous reduction of the penetration depth, which latter is a direct function of the coil diameter with surface coils used for nuclear magnetic resonance measurements. If, however, measurements are to be carried out on human bodies, penetration depths of at least 80 mm are required. This necessitates coil diameters of at least 180 mm.
On the other hand, one always endeavors in the case of nuclear magnetic resonance measurements, especially in the case of imaging methods in nuclear resonance tomography, to have the highest possible measuring frequency. In the before-mentioned example of a coil diameter of 180 mm, the limit where the natural resonance of such a coil is in the range of the operating frequency is already reached at approximately 80 MHz.
Now, it would of course be possible to reduce the inductance of such coils by producing them from broad copper strips; this has to be ruled out, however, for practical reasons since imaging nuclear resonance measurements require the insertion of strong field gradients which would induce correspondingly strong eddy currents in broad copper strips.
For example, it would be possible with the aid of a ring coil made from a copper strip of 30 mm width and a diameter of 150 mm, and using a tuning capacitor of 5 pF, to achieve a penetration depth of approximately 60 mm in no-load condition, in the frequency range of 85 MHz.
In practical measurements, however, it has to be taken into account that biological objects (human bodies), in addition to causing dielectric losses, also have the effect of stray capacitances detuning the resonance circuit. In the described example, such detuning effect may be so important that it can no longer be compensated by usual tuning measures.
In an effort to achieve higher frequencies with an unchanged coil geometry (diameter), one has made use of a trick with the before-mentioned CRC coils, which consists in exciting in a suitable way in a coil arrangement, which normally would oscillate in a low-frequency oscillating mode, a higher frequency mode--and only that--so that a higher measuring frequency is obtained with a relatively big coil diameter.
However, it is a disadvantage of the known CRC coil arrangements that the axial dimensions are very important, due to the absolute necessity to use two ring coils. In addition, any interference, for example any load, produces its effects in both coils, including loads located on the side opposite the measuring object.